High thrust (in excess of 100,000 pounds) expander cycle rocket engines have traditionally been limited to chamber pressures below 1500 psia because of lack of turbine power available to the fuel turbopump. In a typical expander cycle rocket engine, fuel from the fuel turbopump is pumped through the cooling liner and tubular nozzle of the engine's combustion chamber/nozzle assembly where the fuel is heated and then fed to a turbine which drives the fuel turbopump. In order to increase the combustion chamber pressure, fuel flow to the combustion chamber must be increased. However, as fuel flow through the cooling liner and tubular nozzle increases, the temperature of the fuel at the turbopump turbine inlet decreases due to the increase in mass flow rate of the fuel. At the same time, the fuel turbopump must do more work to provide the increased mass flow rate of fuel. Although the energy available to the fuel turbopump turbine is a function of both the mass flow rate of the fuel and the turbine inlet temperature, the increase in the mass flow rate of fuel cannot offset the resulting decrease in turbine inlet temperature which occurs as a result of the increased fuel flow rate. Consequently, the decrease in turbine inlet temperature and the increase in work required by the turbopump at the higher fuel flow rates act to limit the maximum fuel flow rate to the combustion chamber, thereby limiting combustion chamber pressure.
What is needed is an expander cycle rocket engine in which turbine power can be maintained at higher fuel mass flow rates, thereby providing combustion chamber pressures in excess of 1500 psia.